1. Field of the Invention
The present invention relates to an improvement in the door seal arrangement of a microwave oven (heating apparatus).
2. Background of the Invention
Japanese Patent Unexamined Application Publication No. 25190/1985 discloses a proposal in which grooves are formed in a circumferential edge of a door of a microwave oven. The grooves each have a different characteristic impedance in the direction of the depth of the door. The characteristic impedance of the grooves in the direction of the depth is made discontinuous so that the impedance at the inlet of the grooves become a maximum even if the substantial depth is smaller than one-fourth of the wavelength to be used. It thereby becomes possible to reduce the wave leakage similarly to choke grooves. In this example of the prior art, the structure is considerably complicated because grooves are different in width in the direction of groove depth and the shape of the peripheral wall of the grooves is varied in the direction of depth. Further, it is necessary to consider prevention of reflection at portions where the characteristic impedance is discontinuous.
As shown in FIG. 1, Japanese Utility Model Unexamined Application Publication No. 795/1986 discloses a structure in which a cavity resonator 12 having a bent form and having a rectangular section for preventing radiation leakage is provided in the outer circumference of a door 5. An inlet 25 is formed by opposing a cut end plane of an extending surface 11 (which is one of peripheral walls of the cavity resonator 12) to another wall surface (a first wall surface 8) of the cavity resonator 12 to thereby define the inlet 25. In this example of the prior art, waves of higher order modes which may travel not only in the illustrated y-z plane but also in other directions come into the cavity resonator 12. Such oblique waves cause the cavity resonator 12 to not be in a resonant state for the oblique waves to thereby reduce the effect of preventing radiation leakage.
Furthermore, in the conventional example, it is necessary that the size of the section AB of the cavity resonator 12 is large, and therefore the cavity resonator is unsuitable for reduction in size as well as cost of the door.
In FIG. 1 the drawing in the specification of the above-mentioned Japanese Utility Model Unexamined Application Publication No. 61-795 is shown with the same dimensional ratio of the various parts, and the same names and numerical references of the constituent elements corresponding to those of the present invention are used.
As described above, in the conventional microwave oven, there have been problems in that it is necessary to form a groove having a complicated shape, the arrangement of reflection prevention at the characteristic impedance discontinuous portion is troublesome, and the door cannot be reduced in size.